1. Field of the Invention
This invention is concerned with (a) rapidly solidified iron alloys which contain boron and combine high strength with corrosion and oxidation resistance, and, (b) the preparation of these materials in the form of powder and the consolidation of these powders (or, alternatively, the ribbon-like material produced by a rapid solidification process) into bulk parts which are heat treated to a uniform microstructure and desirable properties.
2. Description of the Prior Art
Tool steels have many important metallurgical characteristics in common. In general, metal alloys useful as tool steels exhibit high hardness and resistance to abrasion as well as, for many alloys, the retention of these attributes at high temperatures. These characteristics are obtained by the proper choice of alloy composition, generally iron based with high carbon and alloying metal content.
Tool steels with high hardness and wear resistance are also used as bearing materials. Type M50high speed steel (Fe.sub.bal Cr.sub.4 Mo.sub.4.25 V.sub.1 Mn.sub.0.25 Si.sub.0.2 C.sub.0.8) is widely used in high temperature aircraft bearings but has low corrosion resistance because of the low chromium content of the alloy. Type 440-C, a stainless tool steel [Fe.sub.bal Cr.sub.17 Mo.sub.0.5 Si.sub.0.30 Mn.sub.0.4 C.sub.1.0 ] has poor hot hardness and is usually used at room temperature. The popular bearing steel 52100 (Fe.sub.bal Cr.sub.1.5 Mn.sub.0.35 Si.sub.0.25 C.sub.1.0) has both poor corrosion/oxidation resistance and poor high temperature properties. Another commercial bearing steel, 14-4 Mo (Fe.sub.bal Cr.sub.14 Mo.sub.4 Mn.sub.0.1 Si.sub.0.1 C.sub.1.05) has good high temperature properties and corrosion resistance but is known to have poor hot workability.
Obtaining the desired properties for highly alloyed tool steels depends mainly upon control of the microstructure; generally, desirable properties are obtained from a homogeneous distribution of the carbides in a host structure having a small grain size. The complex chemical composition of tool steel makes the solidification process complicated and leads to coarse multiphase microstructures by following normal solidification procedures. Therefore, these steels possess a natural tendency for compositional segregation. Heterogeneity of structure and composition, particularly of carbide particle size and distribution, is one of the inherent problems in the production of high performance tool steels by conventional practice.
In conventional practice, an as-cast ingot exhibits a microstructure which is then somewhat broken up by hot deformation processes. However, the final product may still exhibit relatively large heterogeneities. Also, because of hot rolling, there is a tendency for grain elongation in the rolling direction and the lining up, or banding of carbide particles, which leads to anisotropic mechanical properties.
In order to minimize these problems, powder metallurgical technologies have recently been applied to the production of tool steels. Conventional powders of tool steels are produced by the atomization of the molten alloy. The faster solidification rate associated with the atomization process, compared to cast ingots, results in particles having a finer microstructure, i.e., having a carbide morphology similar to that of the conventionally cast ingot but with characteristic grain dimensions which are orders of magnitude smaller. Thus, the faster solidification rate decreases the scale of the compositional segregation associated with the solidification to a multiphase microstructure. The powders are subsequently consolidated into parts by conventional powder metallurgical techniques (see "High Speed Tool Steel By Particle Metallurgy" by A. Kasak, G. Steven and T. A. Neumeyer, Society of Automotove Engineers, Automotive Engineering Congress, Detroit, 1972 and "P/M Alternative To Conventional Processing of High Speed Steels" by T. Leven and R. P. Hervey, METALS PROGRESS, Volume 115, No. 6, June 1979, Page 31).
Because of their finer grain size, more uniform dispersion of fine carbides and improved alloy homogeneity, tool steels processed by such powder metallurgical techniques exhibit, compared to cast materials, superior performance a better response to hardening heat treatments, improved dimensional stability and improved hot workability.
During the last two decades, rapid solidification processing (RSP) (also known as rapid liquid quenching (RLQ)) techniques have been used to fabricate new materials having in some cases new and useful properties. In RSP processes, the liquid is typically cooled at rates of .about.10.sup.5 .degree.-10.sup.7 .degree. C./sec and thus solidifies in a very short period of time. The rapid solidification rate leads to a microstructure, and in some cases a metastable atomic structure, different from that obtained from standard solidification procedures. A great deal of research and development effort has been expended on amorphous metals (also referred to as metallic glasses or noncrystalline metals) made by a RSP process. Interesting new crystalline materials, including metastable crystalline phases, alloys having an ultrafine grain size and compositionally homogeneous alloys, can also be made utilizing a RSP process. Further, economical RSP methods for fabricating large quantities of metallic alloys in the form of filaments or strips are well established as the existing state-of-the-art.
Metal powders, when produced directly from the melt by conventional liquid atomization techniques, are usually cooled three to four orders of magnitude faster than a cast ingot although typically still two or more orders of magnitude slower than is possible with RSP techniques. However, improved processes are now being developed for making powders directly from the melt. For example, it has been reported (see D. J. Looft and E. C. Van Reuth; Proc. Conf. on Rapid Solidification Processing, p. 1, Reston, VA. Nov. 1977) that rapidly solidified metal powders can be made at cooling rates in excess of 10.sup.5 .degree. C./sec by centrifugal atomization of a liquid metal stream followed by forced convective cooling. Other approaches of the production of powders at high cooling rates have been reported, for example, that of Murphy and Miller (Scripta Met., Vol. 13, pp. 673-676, 1979).
Because of the potential benefits to be gained, there has been past interest in studying the effects of RSP on tool steels. I. R. Sare and R. W. K. Honeycombe applied RSP to a commercial, molybdenum-rich high speed steel (AISI-Ml containing 8.4% Mo-1.5% W-4.1% Cr-1.1% V-0.77% C) using the method of "gun" splat quenching technique in which molten droplets are impact quenched against a cold metal substrate (see Rapidly Quenched Metals, N. J. Grant and B. C. Giessen, Eds., MIT Press, Cambridge, MA., 1976, pp. 179-187). The quenched high speed tool steel consisted primarily of a two phase mixture of a b.c.c. (.delta.-ferrite) phase and a f.c.c. (austenite) phase. J. Niewiarowski and H. Matyja also found a mixture of two or more phases in rapidly solidified tool steels made by a "piston and anvil" type splat quenching technique (see Rapidly Quenched Metals III, B. Canton, Ed., The Metal Society, 1978, pp. 193-197). However, neither effort produced a homogeneous alloy. Further, neither of the processes which were used is amenable to scale-up for economical commercial production.